1. Field of the Invention
The invention relates to a process and apparatus for preparing trialkoxysilanes of the general formula I
(RO)3SiHxe2x80x83xe2x80x83(I)
in which R is an alkyl group having from 1 to 6 carbon atoms, by reacting silicon with alcohols in an inert solvent in the presence of a copper catalyst.
2. Discussion of the Background
It is known that monomeric hydroalkoxysilanes, such as trimethoxysilane or triethoxysilane, constitute a group of important key compounds in the organic chemistry of silicon.
Hydrosilylation by way of the Sixe2x80x94H function is a route to diverse functional organosilane compounds, examples being alkyl-, aminoalkyl-, haloalkyl-, vinyl-, and epoxyalkylalkoxysilanes. These compounds possess a very extensive field of application.
The monosilanes may be obtained in high purity by base-catalyzed dismutation of hydroalkoxysilanes. The monosilanes are used, for example, in the field of the semiconductor industry.
Hydroalkoxysilanes, such as triethoxysilane (TEOS) or trimethoxysilane (TMOS), are nowadays produced on the industrial scale by esterifying trichlorosilane with ethanol or methanol. Trichlorosilane is obtained in industrial volumes by reacting elemental silicon with hydrogen chloride. The process used in industry for producing TMOS and TEOS involves two stages, trichlorosilane synthesis and subsequent esterification. Because of the known high chemical aggressiveness of hydrogen chloride, both stages require a high level of plant investment and high ongoing costs for the servicing of such a plant. Moreover, the trichlorosilane synthesis yields chlorine-containing residues which are self-igniting and very awkward and expensive to dispose. TEOS or TMOS prepared by this process includes a significant chlorine content. The chlorine fraction in these products can be reduced into the ppm range only by means of very complex distillation processes.
Many fields of use are increasingly requiring the employment of chlorine-free organosilanes. Such silanes are unavailable by the customary preparation processes.
One approach to solving this problem might lie in the copper-catalyzed direct reaction of elemental silicon with alcohols, such as methanol or ethanol, to give TMOS or TEOS. The majority of the processes described, however, possess little technical relevance, despite the fact that the first patent applications in this area go back to as early as 1949 (U.S. Pat. No. 2,473,260). A particular disadvantage is the very low reactivity of the silicon grades used toward methanol or ethanol in a chlorine-free preparation technique, with the consequence that the yields obtained, based on the silicon used, are very low. Moreover, the preparation of the Cu/Si catalyst composition is highly complex, said composition being obtainable, for example, in accordance with the teaching of U.S. Pat. No. 3,641,077 by sintering of copper and silicon at 1050xc2x0 C. followed by ultrafine grinding. Using methanol, after a reaction period of 4 to 5 hours at reaction temperatures of 280xc2x0 C., only about 8% of the silicon used is converted to organosilanesxe2x80x94about 5% to TMOS and about 3% to tetramethoxysilane. Because of the lower reactivity of ethanol, only 6% of the silicon used is converted to organosilanes under otherwise identical reaction conditionsxe2x80x94about 5% TEOS and about 1% tetraethoxysilane. Propanol and butanol are even less chemically reactive; the silicon conversions are 1.5% and 0.7%.
Of the direct dimethyldichlorosilane synthesis by means of a copper-catalyzed reaction of silicon with methyl chloride it is known that organosilanes are obtainable in high yields by the direct synthesis, by using chlorine agents. Apparently this is also the case for the direct synthesis of TEOS and TMOS. Industrially useful TEOS yields of more than 70%, with a selectivity of more than 90%, are described in DE-C 22 47 872. The catalyst used there is CuCl. Additional activation was achieved by using ethanol, with an addition of 0.17% HF. A chlorine-free product is, however, not obtained, owing to the use of CuCl as the catalyst. Moreover, the hydrogen fluoride admixed to the ethanol causes corrosion problems and also occurs in traces in the product.
Measures for further increasing the conversions are reported in European patent no. 0 280 517. There, the silicon powder used is activated beforehand using methyl chloride. The catalyst used is again CuCl. The TMOS conversion rates are stated as being 81% with respect to the silicon used, and the selectivity as being 88%. In the case of the synthesis of TEOS, the reactivity is much reduced: 60.2% silicon conversion with 78.4% selectivity. Again, the process does not operate without chlorine, and the yields are not very attractive for an industrial application.
U.S. Pat. No. 4,727,173 describes the chlorine-free preparation of trimethoxysilane with yields  greater than 80% (based on the silicon used) by reacting methanol with standard commercial silicon powderxe2x80x94purity: 98.5% by weight, Fe less than 0.5% by weight without further activation stepsxe2x80x94under the influence of a copper(II) hydroxide catalyst. Subsequent experiments with standard commercial silicon powder, very finely ground, and Cu(OH)2 catalyst in the synthesis of TMOS led only to unsatisfactory yields of approximately 30%. Furthermore, in comparison to the CuCl-catalyzed reaction an increased fraction of siloxanes was foundxe2x80x94a consequence of the thermal decomposition of Cu(OH)2 to CuO and water, which in turn hydrolyzes the trialkoxysilanes present, eliminating alkanol and forming siloxanes. In addition, a chlorine-free TEOS preparation with standard commercial Cu(OH)2 as catalyst is generally not possible.
It is therefore an object of the invention to develop an economic process for preparing substantially chlorine-free trialkoxysilanes. This object is achieved in accordance with the invention as specified in the claims.